This invention relates to superconductive material and apparatus and systems utilizing such material. Characteristics of preferred compositions herein give rise to apparatus design advantages. Such characteristics include useful values of critical temperature and critical magnetic field. Certain of the compositions are free of characteristics which have been associated with radiation damage in prior materials.
The field of superconductivity has had a varied history from its beginnings—from the discovery of the phenomenon by Kamerlingh Onnes in 1911. Implications of extreme practical significance were apparent from the start. Workers became enchanted with the thought of lossless transmission as well as implications on magnetic and other apparatus. The concept that has received so much attention in recent years—that of “permanent” magnets of extremely high field values useful e.g. for containment of fusion reactions—was not overlooked.
Advances in obtaining material with improved properties have been discontinuous. Experiments following the first discovery, while certainly substantiating the concept, established need for extremely low temperatures, i.e., low values of transition temperatures, Tc. Mercury (Tc˜4 K) was discovered first, and lead (Tc˜7 K) was discovered to be superconducting shortly thereafter.
There was little achievement either experimentally or in theoretical terms following the initial period until the 1940's when work on NbN yielded a Tc value of about 16 K. Evolution and description of the concept was not broadly and effectively disseminated because of its German origin and circumstances surrounding World War II. A significant period of inquiry began subsequent to World War II. From the mechanistic standpoint, the most significant advance entailed identification of Type II superconductivity (Ginzburg and Landau, 20 Zh. Eksperim. i Teor. Fiz., pg. 1064 (1950)).
From a mechanistic-theoretical standpoint 1957 marks a most significant advance in understanding of superconductivity. The Bardeen Cooper Schrieffer Theory (for which a Nobel prize was subsequently awarded), (108 Phys. Rev., pg. 1175 (1957)) gave rise to the understanding which has been implicit in all studies to the present time. Intensive worldwide effort was established by the activity of B. T. Matthias of Bell Laboratories and J. K. Hulm of Westinghouse resulting in the most significant A15 compounds exemplified by Nb3Sn. This was followed by identification of the members as well as of related alloy compositions generally containing Nb. This work gave renewed hope of practical applications working with a refrigerant more effective than liquid helium. Materials developed during this period continue to be studied extensively, and indeed serve very significant technological functions, e.g., in particle accelerators.
Efforts to develop materials with significantly higher values of Tc than that of Nb3Sn(Tc=18 K) were disappointing. Probably the culmination was the Tc value of ˜23.2 K for Nb3Ge in 1973 by L. Testardi at Bell Laboratories and J. R. Gavaler at Westinghouse.
The subsequent decade resulted in little advance in terms of Tc. Extensive study of a new category of material was, however, very important and plays a vital role in development of this invention. This study entailed compositions of barium bismuth lead oxide (BaPb1-zBizO3). B. Batlogg, Physica 126B, 275 (1984). Most significant, compositions in this category while attaining Tc values of only up to 13 K depended upon metal-oxygen bonding for superconductivity.
Most recent origins of the present invention are traceable to the extremely significant work reported by IBM Zurich—J. G. Bednorz and K. A. Muller (64 Z. Phys. B.—Condensed Matter, pp. 189 (1986)). Report of onset Tc values in the 30 K range in the La—Ba—Cu—O system stimulated intense activity by a number of groups worldwide.
Significant consequences of work stimulated by the above are reflected in recent articles.
Workers at the University of Tokyo in Japan J. Appl. Phys., significantly advanced understanding of the IBM work by identification of the superconducting phase as being of the K2NiF4 structure. The composition on which measurements were made in accordance with this publication was that of the IBM Zurich report.
C. W. Chu and co-workers at the University of Houston reported on the beneficial effect of application of hydrostatic pressure during measurement in Phys. Rev. Lett., 58, 405 (1987). In a revision to this article made subsequent to the original submission date, comment was made on the substitution of strontium for barium.
The same issue of Phys. Rev. Lett., 58 at page 408, reported true bulk conductivity at 39.2 K in the composition La1.85Sr0.15CuO4. This finding constitutes a portion of the disclosure in parent U.S. patent application Ser. No. 001,682, filed Jan. 9, 1987.
The published works were accompanied and followed by a series of tantalizing rumors reporting “onset” temperatures well above the 77 K boiling point of liquid nitrogen. The March 2 issue of Phys. Rev. Lett., (Vol. 58, 1987), contains two extremely significant papers representing advances prompted by the new findings. At page 908 et seq., workers at the Universities of Alabama and Houston report on compositions in the Y—Ba—Cu—O system evidencing Tc values well above 77 K. Their exemplary composition (Y0.6Ba0.4)2CuO≦4 is described as multiple phase (“instead of the pure K2NiF4 phase” [associated with previously reported compositions]). Reporting a deviation in resistivity from the usual temperature relationship which is initiated at 93 K, the paper goes on to report attainment of zero resistivity at 80 K. Rather than associated attainment of this value with the particular crystallographic phase, authors state “ . . . high temperature superconductivity may be associated with interfacial manifestations”. At page 911 in the same issue of Phys. Rev. Lett., the same authors in a separate article reached the specific conclusion “It is therefore evident that . . . above 77 K [Tc] may not be identified with . . . perovskite or . . . tetragonal layered structures . . . ” Developments to date have taken on the drama of an international race with runners representing essentially every organization in any way affiliated with superconductivity. Events to date are reflected in an article on page 12 of the Star-Ledger for Tuesday, Mar. 3, 1987. In this article, world-famous physicist John Rowell, Assistant Vice President of Bell Communications Research, reports as their “international contribution” the “ . . . producing results identical to those achieved in general by scientists at the University of Houston . . . ”. Dr. Rowell explicitly confirms the multiphase nature of the superconducting material and, in fact, reports that “Only about 2% . . . is superconducting . . . ”